In recent years, a high efficiency white lighting device in which a semiconductor light-emitting element such as a light-emitting diode (LED) and a phosphor to emit light by being excited with light emitted from the semiconductor light-emitting element are combined has been commercialized. Such a white lighting device using the semiconductor light-emitting element is high efficiency as compared with a conventional white lighting device such as an incandescent lamp or a fluorescent lamp to thus greatly contribute to saving of energy.
Incidentally, in applications of special lighting such as medical lighting, lighting for jewelry goods, lighting for foods, and lighting for photographing, which is necessary to be close to the visibility of colors under natural light, it is required to have a high color rendering property, but the white lighting device using the semiconductor light-emitting element does not necessarily have a sufficient color rendering property. By improving the color rendering property of the white lighting device using the semiconductor light-emitting element, the use of the white lighting device using the semiconductor light-emitting element is made possible even in the applications of special lighting to make it possible to greatly contribute to saving of energy.
As a method of specifying such a color rendering property, an average color rendering index Ra and special color rendering indexes R9 to R15 are cited. When the average color rendering index Ra and the special color rendering indexes R9 to R15 are each close to 100, the color rendering property becomes suitable in the applications of special lighting. Particularly, the special color rendering index R9 is to specify the visibility of red, and it is said that as the special color rendering index R9 is close to 100, the expressiveness of red is high.
In the color rendering method in JIS, lamps called a high color rendering type are classified into three ranks of color rendering A, color rendering AA, and color rendering AAA. In order to substitute the white lighting device using the semiconductor light-emitting element for the lamps with color rendering AA and more among them, the average color rendering index Ra is required to be about 85 or more, and is more preferably required to be 90 or more. Further, in order to express red with good reproducibility, the special color rendering index R9 is required to be about 85 or more, and is more preferably required to be 90 or more.
The white lighting devices each using the above-described semiconductor light-emitting element, particularly a light-emitting diode, are roughly classified into ones each using a blue light-emitting diode emitting blue light and ones each using a near-ultraviolet light-emitting diode emitting ultraviolet light.
As the white lighting devices each using the blue light-emitting diode among them, one combined with a yellow phosphor, and one combined with a green phosphor and a red phosphor are cited. However, the one combined with the yellow phosphor has difficulty in obtaining the average color rendering index Ra of 85 or more, and has difficulty in being used in the applications of special lighting. Further, the one combined with the green phosphor and the red phosphor can obtain the average color rendering index Ra or 85 or more, but has difficulty in obtaining the special color rendering index R9 of 85 or more and has difficulty in being used in the applications of special lighting.
On the other hand, as the white lighting devices each using the near-ultraviolet light-emitting diode, one combined with a blue phosphor, a green phosphor, and a red phosphor, and one combined with a blue phosphor, a red phosphor, and a yellow phosphor in place of the green phosphor have been known. According to the white lighting devices as above, the one having a correlated color temperature in excess of 5000 K can obtain the average color rendering index Ra and the special color rendering index R9 that are 85 or more, and can be used in the applications of special lighting. However, the white lighting device having the correlated color temperature of 5000 K or less has difficulty in obtaining the average color rendering index Ra and the special color rendering index R9 that are 85 or more, and particularly has difficulty in obtaining the special color rendering index R9 of 85 or more, and has difficulty in being used in the applications of special lighting.